This invention relates to a differential amplifier circuit, which has two differential amplifier sections and selectively provides one of the output signals of these sections.
FIG. 1 shows a prior art differential amplifier circuit with an equalizing characteristic switching function, which is used as a preamplifier in a tape recorder. The operational state of this amplifier circuit is set to provide flat frequency characteristics in a recording mode and provide frequency characteristics defined by the NAB standards or like standards in a playback mode.
The differential amplifier circuit includes a differential amplifier DA1 constituted by a pair of npn transistors TR1 and TR2 and another differential amplifier DA2 constituted by a pair of npn transistors TR3 and TR4. The transistors TR1 and TR3 have their bases commonly connected to an input terminal IN1 and their collectors commonly connected to a power supply terminal VC. The transistors TR2 and TR4 have their collectors commonly connected through a resistor R1 to the power supply terminal VC and also commonly connected through an output amplifier 2 to an output terminal OUT1. The transistors TR1 and TR2 have their emitters grounded through a constant current source 4 and a switch SW1. The transistors TR3 and TR4 have their emitters grounded through a constant current source 6 and a switch SW2. The switches SW1 and SW2 are set in mutually opposite switching positions. The transistor TR2 has a base connected through a resistor R2 to the output terminal of the output amplifier 2 and also grounded through a series circuit of a resistor R3 and a capacitor C1. The transistor TR4 has a base connected, through a parallel circuit of a resistor R4 and a capacitor C2 and a resistor R5, to the output terminal of the output amplifier 2 and also grounded through a series circuit of a resistor R6 and a capacitor C3. The resistors R2 and R3 and capacitor C1 form a feedback path having flat frequency characteristics. The resistors R5, R4 and R6 and capacitors C2 and C3 form a feedback path having frequency characteristics defined by the NAB standards. When the switch SW1 is "on", the differential amplifier circuit of FIG. 1 provides flat frequency characteristics, and when the switch SW2 is "on", it provides the frequency characteristics defined by the NAB standards.
It is now assumed that the switches SW1 and SW2 are respectively "on" and "off" so that the constant current source 4 supplies a constant current as shown in FIG. 2 (A) while a constant current from the constant current source 6 is interrupted as shown in FIG. 2 (B), that is, the differential amplifier DA1 is operative while the differential amplifier DA2 is inoperative. In this situation, the DC base potentials of the transistors TR1 and TR2 of the differential amplifier DA1 are substantially at an equal potential, and a base current flows through the resistor R2 to the base of the transistor TR2. The DC potential on the output terminal OUT1 is thus held at a level equal to the sum of the base potential of the transistor TR2 as shown in FIG. 2 (D) and the voltage drop across the resistor R3 due to the base current, as shown in FIG. 2 (C). In the differential amplifier DA2, the transistor TR4 carries no current, so that the base thereof is held at a fixed potential level equal to the DC potential on the output terminal OUT1 as shown in FIG. 2 (E). The capacitor C3 is charged to a voltage corresponding to the DC potential level on the output terminal OUT1.
When the switches SW1 and SW2 are switched to "off" and "on", respectively, the differential amplfifiers DA1 and DA2 are rendered inoperative and operative, respectively. At this time, the base potential of the transistor TR4 is set at a level higher than the base potential of the transistor TR3 due to the charge in the capacitor C3. More specifically, the base potential of the transistor TR4 is set at a level higher than that of the transistor TR3 by the voltage drop .DELTA.VB across the resistor R2 which occurs before the switching action due to the current flowing to the base of the transistor TR2. This voltage .DELTA.VB is amplified to (R4+R5+R6)/R6 times .DELTA.VB by the differential amplifier DA2 and then supplied through the amplifier 2 to an external loudspeaker circuit (not shown). An impulse sound is thus generated from the loudspeaker circuit.
As the capacitor C3 is subsequently discharged, the base potential on the transistor TR4 is reduced to a predetermined level as shown in FIG. 2 (E) and held at this level.
FIG. 3 shows a prior art differential amplifier circuit, which can selectively amplify one of two input signals. Like the differential amplifier circuit shown in FIG. 1, this amplifier circuit includes a differential amplifier DA1 having npn transistors TR1 and TR2, and another differential amplifier DA2 having npn transistors TR3 and TR4, constant current sources 4 and 6 and switches SW1 and SW2. The transistors TR1 and TR3 have their bases connected through capacitors C4 and C5 to input terminals IN2 and IN3 and also connected through resistors 27 and 28 to the positive terminal of a power supply E1, respectively. The transistors TR2 and TR4 have their bases grounded through a resistor R9 and a capacitor C6 and also commonly connected through a resistor R10 and a capacitor C7 to an output terminal OUT1. The transistors TR1 and TR3 have collectors commonly connected to the collector of a pnp transistor TR5 whose emitter is connected to a power supply terminal VC, and also to the base of a pnp transistor TR6 whose emitter is connected to the power supply terminal VC. The transistors TR2 and TR4 have collectors commonly connected to the collector and base of a pnp transistor TR7 whose emitter is connected to the power supply terminal VC and whose base is connected to the base of the transistor TR5. The transistor TR6 has a collector connected to the base of an npn transistor TR8 whose collector is connected to the power supply terminal VC, and is also connected through a resistor R11 to the emitter of the transistor TR8. The emitter of the transistor TR8 is further connected to the connection point between a resistor R10 and a capacitor C7 and also to a constant current source 8.
When the switch SW1 is "on", the differential amplifier circuit amplifies the input signal fed to the input terminal IN2. When the switch SW2 is "on", it amplifies the input signal fed to the input terminal IN3. When the switch SW1 is "on", the differential amplifier DA1 is operative, and a base current flows through the resistor R7 into the base of the transistor TR1. The base of the transistor TR1 is thus held at a DC base potential equal to the difference between the output voltage V0 of the power supply E1 and the voltage drop across the resistor R7. Also, the base potential of the transistor TR3 is held at a level equal to the voltage V0, and the base potentials of the transistors TR2 and TR4 are set equal to the DC base potential of the transistor TR1. When the switches SW1 and SW2 are set to "off" and "on" positions respectively, different voltages are applied to the bases of transistors TR3 and TR4, and the difference between these base potentials is amplified by the differential amplifier DA2. The amplified voltage is supplied through an output circuit constituted by the transistors TR6 and TR8 to the output terminal OUT2.
In the amplifier circuits of FIGS. 1 and 3, the switching of the switches SW1 and SW2 causes a sudden change of the DC voltage on the output terminal, so that an impulse sound will be produced from the loudspeaker circuit connected to the output terminal as mentioned earlier.